1. Technical Field
The present invention relates to a switching power source device capable of shortening the reactivation time during a latch protection operation based on a latch signal that stops the operation of a power source device itself.
2. Related Art
For example, as illustrated in FIG. 3, a switching power source device that switches an input voltage using a switching element such as an insulated gate bipolar transistor (IGBT) or a metal-oxide semiconductor field-effect transistor (MOSFET) to obtain a direct current (DC) output voltage includes a switching power supply main body 10 constituted mainly of a switching element Q and a control circuit 20 that is configured as an integrated circuit, for example, and performs on/off-driving of the switching element Q. The switching power supply main body 10 includes the switching element Q connected in series to an input power supply (not illustrated) through a primary coil Tw1 of a transformer T, for example, and a secondary-side circuit S connected to a secondary coil Tw2 of the transformer T. The secondary-side circuit S includes a rectification circuit that rectifies a voltage generated in the secondary coil S of the transformer T, an output capacitor that smoothes the rectified output to obtain a predetermined DC output voltage, and the like.
The control circuit 20 basically includes a driver circuit 21 that performs on/off-driving of the switching element Q according to a PWM control signal for controlling an ON width of the switching element Q in response to the DC output voltage of the secondary-side circuit S. Although the PWM control signal will not be described in detail in this specification because it is not directly related to the present invention, the PWM control signal is generated in an IC control block 22 that includes an oscillator that determines a switching frequency of the switching element Q and a protection circuit and the like for protecting the switching element Q from an overload, an overcurrent, and the like.
The control circuit 20 includes an activation switch circuit 23 that receives, at its input terminal VH, an input voltage applied to the primary coil Tw1 of the transformer T, for example, during activation of the switching power supply and charges a capacitor C connected to a control power terminal VCC of the control circuit 20. As illustrated in FIG. 4 illustrating an operation timing during activation of the switching power source device, the activation switch circuit 23 is turned on (conductive) during activation to charge the capacitor C until the voltage of the control power terminal VCC reaches an operation start voltage UVLO-on (see Section 1 in FIG. 4).
When the voltage (control power voltage Vcc) of the control power terminal VCC reaches the operation state voltage UVLO-on due to charging of the capacitor C, the IC control block 22 starts operating and the driver circuit 21 is driven under the control of the IC control block 22, and thereby the switching element Q starts a switching operation. After the switching element Q starts the switching operation, a coil voltage occurring in an auxiliary coil Tw3 of the transformer T is applied to the control power terminal VCC through the capacitor C as the control power voltage Vcc, and as a result, the control circuit 20 continues its operation.
When an abnormality such as an overcurrent or an overload is detected and a protection signal is output, the switching operation of the switching element Q is temporarily stopped to protect the switching element Q and the like from the abnormality as illustrated in FIG. 4. Since no voltage is generated in the auxiliary coil Tw2 due to the stopped switching operation of the switching element Q, and no electric power is supplied to the capacitor C, the voltage Vcc of the control power terminal VCC decreases gradually.
In this case, the IC control block 22 turns on/off the activation switch circuit 23 intermittently to charge the capacitor C in order to secure the voltage of the control power terminal VCC (see Section 2 in FIG. 4). By the intermittent charging of the capacitor C in this protection operation period, the control power voltage Vcc is held at a voltage in which the operation function of the IC control block 22 can be maintained without decreasing to an operation stop voltage UVLO-off. The protection operation state continues due to the maintained operation function of the IC control block 22.
After that, when the abnormality factor is eliminated and the protection signal disappears, the control circuit 20 is reactivated, and the switching operation of the switching element Q restarts under the control of the IC control block 22 (see Section 3 in FIG. 4). However, in this case, due to the supply of the driving current to the switching element Q, naturally, the voltage (the control power voltage Vcc) of the capacitor C temporarily decreases. In order to suppress a temporary decrease in control power voltage Vcc as much as possible and to stably secure the voltage equal to or higher than the operation stop voltage UVLO-off, a relatively high-capacity capacitor (for example, approximately 22 μF) is used as the capacitor C.
However, when an abnormality severer than the overload or the overcurrent (for example, an overheat of the switching element Q) is detected, a latch signal is emitted in order to immediately stop the driving of the switching power source device to prevent thermal fracture of the switching element Q. The latch circuit 25 performs a role of forcibly stopping the operation of the driver circuit 21 by being set in response to the latch signal. The protection operation based on such a latch signal continues until the latch circuit 25 is reset and is referred to as a latch protection operation.
As illustrated in FIG. 5 illustrating the operation timing during the latch protection operation, the control power voltage Vcc (the charge voltage of the capacitor C) gradually decreases with the stopped driving of the switching element Q. When the control power voltage Vcc decreases to the operation stop voltage UVLO-off, a comparator 24 detects this state to reset the latch circuit 25 thereby reactivating the control circuit 20. The time required for securing the control power voltage Vcc after the latch protection operation is performed is referred to as a reactivation time (power reactivation time) from the latch protection operation, and it is generally preferable to secure the voltage in a very short time (for example, in 2 seconds).
As a method of shortening the reactivation time in a protection operation based on the overvoltage detection, a method of releasing an overvoltage operation inhibiting signal when an input voltage becomes smaller than a predetermined threshold voltage is proposed (for example, see JP 2009-165288 A).